Equine Grass Sickness

As we are in the high risk season for Equine Grass Sickness it is essential for horse owners to be aware of the disease and the threat it poses to their horse.

Equine Grass Sickness – what is it?

Equine Grass Sickness (EGS) is a debilitating and often fatal disease affecting horses, ponies and donkeys, with a mortality rate in excess of 85%. EGS was first described in eastern Scotland in the early 1900s, and Britain continues to have the highest incidence of EGS worldwide. In Britain, approximately one case occurs in every 50 horses and ponies residing on EGS-affected premises each year, with high risk areas recognised throughout the country.

What to look out for

EGS is characterised by extensive nerve damage, with the gastrointestinal tract being the most severely affected body system. The disease can occur in acute, subacute and chronic forms, reflecting the severity and the duration of clinical signs.

The clinical signs of acute EGS are severe, appear suddenly and may include depression, colic, excessive salivation, difficulty swallowing or inappetance. Clinical signs of subacute EGS are very similar but less severe. Unfortunately, cases of acute and subacute EGS are invariably fatal.

Approximately one-third of all EGS cases present with the chronic form, which has a more gradual onset of clinical signs, most notably profound weight loss. Chronic EGS cases may also develop a “tucked up” abdominal appearance and an abnormal stance. While recovery may be possible in up to 70% of horses with chronic EGS, the level of nursing they require is very intensive and treatment can be prolonged and expensive.

If you notice any of these signs in your horse, the Animal Health Trust recommends calling your vet without delay.

What causes it?

Almost all cases of EGS occur in horses with access to grazing. Growing scientific evidence suggests that the disease may be caused by the bacterium Clostridium botulinum type C, commonly found within soil and capable of producing neurotoxins to which horses are particularly sensitive. The disease is thought to occur when a combination of risk factors triggers the production of these neurotoxins by C. botulinum bacteria already present within the horse’s intestinal tract.

Decades of research have identified an array of risk factors associated with EGS, and it is likely that a combination of several of these factors plays a role in causing the disease. However, many of these risk factors cannot be controlled, including those related to the premises, season and climate, and even those related to the individual horse and pony.

How can we prevent it?

With current evidence-based recommendations not guaranteeing protection from EGS, attention has turned to focus on another method of prevention – vaccination.

The possible association between EGS and C. botulinum means the disease could be similar to other equine clostridial diseases, such as tetanus and botulism, which are both prevented successfully by vaccination. Horses with EGS have lower antibody levels to C. botulinum, and those with higher antibody levels have a reduced risk of disease. Furthermore, horses that have been in contact with an EGS case appear less likely to develop the disease, suggesting they may acquire some degree of immunity. Vaccination against the bacterium is therefore a promising avenue to explore in the possible prevention of EGS.